1. Field of the Invention
The present invention relates to nonreciprocal circuit devices used in a high frequency band such as a microwave band, for example, isolators or circulators. In addition, the invention relates to communication apparatuses incorporating the nonreciprocal circuit devices.
2. Description of the Related Art
In conventional nonreciprocal circuit devices such as lumped-constant isolators and circulators, attenuation in a signal propagation direction is extremely small, whereas attenuation in the opposing direction is extremely large. Conventional nonreciprocal circuit devices having such characteristics are widely used in communication apparatuses to allow oscillators and amplifiers to act in a stable manner and secure functions of the oscillators and amplifiers.
FIG. 8 shows an exploded perspective view of a conventional isolator, and each of FIGS. 9A and 9B show the inner structure of the isolator. FIG. 10 shows an equivalent circuit thereof.
As shown in each of FIG. 8 and FIGS. 9A and 9B, in the lumped-constant isolator, inside a magnetic closed circuit formed by an upper yoke 2 and a lower yoke 8 are arranged a magnetic assembly member 5 composed of central conductors 51, 52, and 53, a ferrite member 54, a permanent magnet 3, and a resin frame 7. Port P1 of the central conductor 51 is connected to an input/output terminal 71 and a matching capacitor C1 and port P2 of the central conductor 52 is connected to an input/output terminal 72 and a matching capacitor C2. The input/output terminals 71 and 72 and the matching capacitors C1 and C2 are disposed in the resin frame 7. Port P3 of the central conductor 53 is connected to a matching capacitor C3 and a termination resistor R. One end of each of the capacitors C1, C2, and C3, and the termination resistor R is connected to grounds 73.
In the equivalent circuit shown in FIG. 10, the ferrite member has a disk shape and a direct-current magnetic field is indicated by the symbol H. The central conductors 51, 52, and 53 are shown as equivalent inductors L. With such a circuit structure, forward-direction characteristics are equivalent to the characteristics of a band pass filter. In frequency bands distant from the pass bandwidth, even in the forward direction, signals are slightly attenuated.
In general, in a conventional communication apparatus, an amplifier used in a circuit of the apparatus always causes distortions to some extent. This is a factor producing spurious components including the second harmonic and the third harmonic of a fundamental wave, by which unnecessary radiation is generated. Since such unnecessary radiation emitted from the communication apparatus causes the malfunction of a power amplifier and interference, standards and regulations for manufacturing the apparatus are pre-determined. Thus, it is necessary to suppress the unnecessary radiation below a certain level. In order to prevent unnecessarily radiation, it is effective to use an amplifier having good linearity. However, since such a amplifier costs much, with the use of a filter or the like, usually, unnecessary frequency components are attenuated. However, still, such a filter costs and the size of the apparatus increases. In addition, there is a loss generated by the filter.
Therefore, it is considerable to suppress spurious components by using characteristics of a band pass filter included in an isolator or a circulator. However, it is impossible to obtain sufficient attenuation characteristics in unnecessary frequency bands by using the conventional nonreciprocal circuit device having a basic structure shown in each of FIGS. 8 to 10.
In order to solve the above problems and obtain a large amount of attenuation in spurious frequency bands including the second harmonic and third harmonic of a fundamental wave, there is disclosed a nonreciprocal circuit device in Japanese Unexamined Patent Application Publication No. 10-93308. Each of FIGS. 11, 12A and 12B, and 13 shows an isolator as an example of the nonreciprocal circuit device. FIG. 11 shows an exploded perspective view of the isolator Each of FIGS. 12A and 12B shows the inner structure of the isolator, and FIG. 13 shows an equivalent circuit of the isolator.
Unlike the isolator shown in each of FIGS. 8 to 10, the isolator includes an inductor of for a band pass filter. The inductor Lf is connected between the port P1 of a central conductor 51, a matching capacitor C1, and an input/output terminal 71. As the inductor, a solenoid coil adaptable to miniaturization of the device is used. An isolator applied for the 900-MHz band uses a coil having an inductance of approximately 24 nH. More specifically, a coil used in this isolator is formed by a copper wire having a diameter xcfx86 of 0.1 mm which is wound 9 turns with an external diameter xcfx86 of 0.8 mm.
A capacitor Cf is connected in series to the input/output terminal 71 of the isolator having the above structure. With this connection, as in the equivalent circuit shown in FIG. 13, the capacitor Cf and the inductor Lf form a band pass filter, with the result that the signal components of frequencies distant from the pass bandwidth can be attenuated.
FIG. 14 shows a graph illustrating frequency characteristics of the isolator (a first conventional example) shown in FIGS. 8 to 10 and the isolator (a second conventional example) shown in FIGS. 11 to 13. This graph shows the frequency characteristics of the isolators applied for the 900-MHz band. When compared with the first conventional isolator, in the second conventional isolator, attenuation or the second harmonic (1800 MHz) is improved from 19.3 dB to 28.3 dB, and attenuation of the third harmonic (2700 MHz) is improved from 28.6 dB to 40.1 dB.
As described above, when the inductor is disposed in the nonreciprocal circuit device to form a filter permitting attenuation of unnecessary frequency components, the entire circuit structure can be made smaller than the structure including a single filter disposed outside of the device.
Recently, with an increasing need for further miniaturization of a mobile communication apparatus, there has been a demand for a more compact nonreciprocal circuit device incorporating an inductor for a filter. Thus, it is also necessary to reduce the size of such an inductor. However, when a solenoid inductor is miniaturized, inductance of the inductor becomes smaller, thereby reducing attenuation in the second harmonic and third harmonic of the fundamental wave. In addition, in order to miniaturize such a solenoid inductor without decreasing inductance, it is possibly considerable to provide a solenoid within a magnetic member. However, this arrangement newly requires a magnetic member, and manufacturing of the structure is a difficult task, which increases cost.
Accordingly, it is an object of the present invention to provide a compact nonreciprocal circuit device in which a large amount of attenuation can be obtained at a predetermined frequency band without increasing cost. It is another object of the invention to provide a communication apparatus using the nonreciprocal circuit device.
The present invention provides a nonreciprocal circuit device including a magnetic member to which a direct current magnetic field is applied, the magnetic member including a plurality of central conductors arranged to intersect one another, and a series resonant circuit including a capacitor and an inductor. The series resonant circuit is connected between at least one of the central conductors and a ground, and has a resonance frequency greater than the central frequency of a pass bandwidth of the nonreciprocal circuit device. The series resonant circuit is formed by directly connecting a cold end of the capacitor and a hot end or the inductor.
Regarding a communication apparatus, the frequencies of major problematic spurious components generated are higher than a basic wave frequency. Thus, when a series resonant circuit as a trap filter having a resonance frequency higher than the central frequency (hereafter referred to as a xe2x80x9cbasic wave frequencyxe2x80x9d) of a pass bandwidth of the nonreciprocal circuit device is connected between the central conductor and the ground, the spurious signals of frequencies higher than the basic wave frequency flow to the ground via the series resonant circuit. As a result, the spurious components passing through a signal path are attenuated. Usually, the higher the resonance frequency becomes, the smaller the resonant circuit can be made. Thus, when the resonant circuit resonates with the spurious components of frequencies higher than the central frequency to selectively attenuate the spurious components, the resonant circuit can be made smaller than the series resonant circuit resonating with the central frequency on the signal path to selectively pass the signal components as in the case of the conventional nonreciprocal circuit device shown in each of FIGS. 11 to 13. As a result, an inductor forming the series resonant circuit can be disposed on the cold-end side of a capacitor conventionally used for matching to be directly connected to the cold end of the capacitor. In this arrangement, while the inductor can be efficiently contained in the nonreciprocal circuit device, the size of the device can be reduced.
In the nonreciprocal circuit device, the inductor may be a chip inductor. By arranging the chip inductor on the cold-end side of the capacitor, the assembling and connection of components can be facilitated. Thus, simplification of the manufacturing process and cost reduction can be achieved.
In addition, the present invention provides a nonreciprocal circuit device including a magnetic member to which a direct-current magnetic field is applied, the magnetic member including a plurality of central conductors arranged to intersect one another, a series resonant circuit including a capacitor and an inductor, the series resonant circuit being connected between at least one of the central conductors and a ground terminal and having a resonance frequency greater than the central frequency of a pass bandwidth of the nonreciprocal circuit device, and a resin frame for containing the capacitor. In this nonreciprocal circuit device, the inductor is formed by insert-molding an electrode thereof in the resin frame. With this arrangement, since the inductor is integrally formed with the resin frame containing the capacitor, the number of the components can be reduced by one, thereby leading to simplification of the manufacturing process and cost reduction. Moreover, by connecting the cold end of the capacitor to the hot end of the inductor, further simplification of the manufacturing process can be achieved.
In addition, the present invention provides a nonreciprocal circuit device including a magnetic member to which a direct-current magnetic field is applied, the magnetic member including a plurality of central conductors arranged to intersect one another, a series resonant circuit including a capacitor and an inductor, the series resonant circuit being connected between at least one if the central conductors and a ground terminal and having a resonance frequency greater than the central frequency of a pass bandwidth of the nonreciprocal circuit device, and a yoke forming a closed magnetic path. The inductor is formed by cutting a portion of the yoke. With this arrangement, since the inductor is integrally formed with the yoke, the number of components is reduced by one. As a result, the manufacturing process can be simplified and the cost can be reduced. In addition, the cold end of the capacitor may be connected to the hot end of the inductor. Thus, further simplification of the manufacturing process can be achieved.
In the nonreciprocal circuit device, the inductor and the ground terminal may be integrally formed with the same member. With this arrangement, since it is unnecessary to use another member as the ground terminal, the number of used components can be reduced and the distance between the cold end of the inductor and the ground terminal can be shortened. As a result, increase in unnecessary impedance can be suppressed.
Furthermore, in the nonreciprocal circuit device, series resonant circuits may be disposed between two or more central conductors and ground terminals. This increases attenuation of spurious components and permits the spurious components to be attenuated in a broad band.
Furthermore, in the nonreciprocal circuit device, of the two or more series resonant circuits, at least one series resonant circuit may have a resonance frequency different from a resonance frequency of the remaining series resonant circuit. With this arrangement, spurious components generated in a broad frequency band or in a plurality of frequency bands can be attenuated.
Furthermore, in the nonreciprocal circuit device, at least one of the two or more resonant circuits may have a resonance frequency that is substantially twice the central frequency of the pass bandwidth, and at least another resonant circuit may have a resonance frequency that is substantially three times the central frequency of the pass bandwidth.
The major problematic spurious components generated in a communication apparatus are spurious components such as the second harmonic and third harmonic of a basic wave frequency. In order to remove such spurious components distributed in a plurality of frequency bands distant from the basic wave frequency by using a trap filter, the resonance frequency of at least one of the plurality or series resonant circuits is set to be substantially twice the central frequency or the pass bandwidth, whereas the resonance frequency of at least another series resonant circuit is set to be substantially three times the central frequency of the pass bandwidth. In this manner, by matching the resonance frequency of each series resonant circuit with the frequencies of the second harmonic and the third harmonic, spurious components can be efficiently attenuated. In this case, the frequencies of xe2x80x9csubstantially twicexe2x80x9d are included in a range from approximately 1.5 to 2.5 times the central frequency of the pass bandwidth. The frequencies of xe2x80x9csubstantially three timesxe2x80x9d are included in a range from approximately 2.5 to 3.5 times the central frequency of the pass bandwidth.
Furthermore, in the nonreciprocal circuit device, an equivalent capacitance to the series resonant circuit at the central frequency of the pass bandwidth may be set as a matching capacitance with respect to the central frequency of the pass bandwidth.
Since the resonance frequency of each series resonant circuit is set to be greater than the central frequency, the resonant circuit acts as a capacitive impedance with respect to the central frequency. Thus, by appropriately designing the inductor and capacitor forming the series resonant circuit, an equivalent matching capacitance with respect to the central frequency can be obtained. With this arrangement, when the series resonant circuit is disposed as a trap filter, it is unnecessary to dispose another matching capacitor. Thus, since the number of used components can be reduced, miniaturization of the device and cost reduction can be achieved.
In addition, the present invention provides a communication apparatus incorporating the nonreciprocal circuit device according to the invention. In this communication apparatus, for example, the nonreciprocal circuit device is disposed as a circulator for dividing transmitted signals and received signals. With this arrangement the communication apparatus of the invention can be made compact while having satisfactory spurious characteristics.